Heretofore, in order to improve the quality of formed glass products, a refining step is carried out to remove bubbles generated in molten glass produced by melting a raw material in a melting furnace, before shaping the molten glass in a forming apparatus.
In this refining step, there has been known a method of adding e.g. sodium sulfate (Na2SO4) as a refining agent into a raw material in advance, melting the raw material to produce molten glass, retaining and maintaining the molten glass at a predetermined temperature, and thereby making bubbles in the molten glass grow and move up by the refining agent, to remove these bubbles.
Further, there has been known a vacuum degassing method of introducing molten glass in a vacuum atmosphere, growing bubbles present in a flow of molten glass continuously flowing in the vacuum atmosphere, thereby making the bubbles move up and break to remove the bubbles, followed by exhausting the molten glass from the vacuum atmosphere.
In the vacuum degassing method, a molten glass flow is formed and the molten glass moves in a vacuum atmosphere, specifically, in a vacuum degassing vessel inside of which is maintained to a predetermined vacuum degree. When the molten glass moves in the vacuum degassing vessel, bubbles contained in the molten glass are grown remarkably relatively in a short time, so that the grown bubbles move up in the molten glass by their buoyance forces and destroyed at a surface of the molten glass, thereby to remove the bubbles from the molten glass surface efficiently.
In such a vacuum degassing apparatus, the material constituting a conduit for molten glass such as a vacuum degassing vessel, an uprising pipe or a downfalling pipe, that constitutes a flow path for molten glass, is required to be excellent in heat resistance and corrosion resistance against molten glass. As a material satisfying this requirement, platinum or a platinum alloy such as a platinum-rhodium alloy, or a refractory bricks such as electrocast bricks, are employed.
These materials are materials excellent in heat resistance and corrosion resistance against molten glass, but in each of the cases where the conduit for molten glass is made of refractory bricks, platinum or a platinum alloy, bubbles may be generated on an interface between a conduit wall face and molten glass. When such generation of bubbles on the interface between the conduit wall face and the molten glass occurs in a vacuum degassing vessel (particularly on the downstream side of the vacuum degassing vessel) or in a downfalling pipe, it is difficult to remove bubbles from the molten glass, which causes defects in product glasses.
As described above, in order to remove bubbles in molten glass efficiently and securely, a process of growing bubbles in a molten glass and making the bubbles move up and break on a surface of the molten glass, is necessary. In order to conduct such a process securely and efficiently, it is necessary to maintain the degree of vacuum in the vacuum degassing vessel within a proper range.
In the vacuum degassing method for molten glass described in Patent Document 1, in order to always maintain the degree of vacuum in a vacuum degassing vessel within a proper range, it is proposed to compensate the degree of vacuum in the vacuum degassing vessel in accordance with change of a barometric pressure. However, when the degree of vacuum in the vacuum degassing vessel is compensated, the level of molten glass in the vacuum degassing vessel changes to affect the effect of vacuum degassing. Accordingly, in the vacuum degassing method for molten glass described in Patent Document 1, when the degree of vacuum in the vacuum degassing vessel is compensated, it is proposed to move up and down the position of the vacuum degassing vessel to maintain the level of molten glass in the vacuum degassing vessel to be constant.
In the method described in Patent Document 1, while the level of molten glass in the vacuum degassing vessel is maintained to be constant, the degree of vacuum in the vacuum degassing vessel is always maintained within a proper range, whereby it is possible to maintain the effect of vacuum degassing always in an optimum condition.
However, it is not possible to move up and down a vacuum degassing vessel in every vacuum degassing apparatus. For example, in a case of employing a large-sized vacuum degassing vessel to increase the degassing capacity of molten glass, it is extremely difficult to move up and down such a vacuum degassing vessel in accordance with compensation of the degree of vacuum in the vacuum degassing vessel, and such a method is not practical.
Further, in a case of a vacuum degassing apparatus having a structure that an uprising pipe and a downfalling pipe are fixed to an upstream side pit and a downstream side pit, respectively, such as the vacuum degassing apparatus described in Patent Document 2, it is not possible to move a vacuum degassing vessel up and down.
In such cases of a vacuum degassing apparatus having a vacuum degassing vessel that cannot be moved up and down, when the degree of vacuum in the vacuum degassing vessel is compensated in accordance with change of barometric pressure, the level of molten glass in the vacuum degassing vessel changes to affect the effect of vacuum degassing. Particularly, when the level of molten glass in the vacuum degassing vessel rises, the distance from the bottom of the vacuum degassing vessel to the level of molten glass increases, which prevents bubbles present in the vicinity of the bottom of the vacuum degassing vessel from moving up, and decreases the effect of vacuum degassing. When a vacuum degassing vessel cannot be moved up and down, it is difficult to adjust the pressure at the bottom since it is determined by the depth of the bottom from the level of molten glass in the vacuum degassing vessel.    Patent Document 1: JP-A-2006-306662    Patent Document 2: JP-A-2000-7344